1. Field of the Invention
This invention relates to an information recording medium. This invention particularly relates to an information recording medium, which is provided with a layer of a ferroelectric substance and on which information is recorded by means of directions of polarization of the ferroelectric substance. This invention also relates to a method for recording information on the information recording medium.
2. Description of the Prior Art
As information recording media, on which various kinds of information, such as image signals and audio signals, are recorded or which are used as signal memories for computers, magnetic recording media, magneto-optical recording media, or the like, have heretofore been used widely.
Also, an information recording medium, on which the information can be recorded and erased, from which the information can be reproduced, and on which the information can be recorded at a high density, has been proposed in, for example, Japanese Unexamined Patent Publication No. 57(1982)-27447. The proposed information recording medium comprises a semiconductor layer and a layer of a ferroelectric substance overlaid on the semiconductor layer and records the information by means of directions of polarization of the ferroelectric substance. Recording of the information on the proposed information recording medium is carried out by moving an electroconductive head on the ferroelectric substance layer, and at the same time applying a voltage to the ferroelectric substance layer such that only predetermined portions of the ferroelectric substance layer may be selectively polarized in a predetermined direction. When the information is to be reproduced from the information recording medium, a change in the electrostatic capacity of the recording medium due to depletion layers, which are formed in the semiconductor layer by the polarization of the ferroelectric substance, is detected by an electroconductive head.
As the electroconductive head for reproduction, theoretically, a reproducing head, which is employed in a video disk system referred to as the VHD system, can be used. Specifically, with the video disk for the VHD system, the depletion layers described above are not formed, but a change in the electrostatic capacity is obtained from the presence or absence of pits. Therefore, the head for reproducing the recorded information is constituted so as to detect a change in the electrostatic capacity, which change occurs in accordance with the recorded information.
It is considered that a coercive field Ec serves as one of indexes for the stability of the information, which has been recorded by means of the directions of polarization of the ferroelectric substance. The stability of the information recorded on the information recording medium will hereinbelow be studied from the point of view of the coercive field Ec.
As for the conventional information recording medium disclosed in Japanese Unexamined Patent Publication No. 57(1982)-27447, the material which is considered for use as the ferroelectric substance is only lead titanate, which is an inorganic material. The coercive field Ec of the inorganic ferroelectric substance having a perovskite structure, such as lead titanate, is approximately on the order of several megavolts/m. In cases where the inorganic ferroelectric substance is used to record information at a high density, it is necessary for the thickness of the layer of the ferroelectric substance to be reduced to a value approximately equal to the recording spot length, e.g., to approximately 0.1 .mu.m. In such cases, the coercive field Ec can be converted to a voltage on the order of 0.1 V. Specifically, if a voltage higher than this order is applied to the ferroelectric substance layer, the recorded signal will disappear.
When a signal is reproduced from the information recording medium, in general, if the amplitude of a radio-frequency (RF) voltage, which is applied to the resonance circuit containing the information recording medium via the electroconductive head during the detection of the electrostatic capacity of the information recording medium, is lower than 1V, a reproduced signal will be obtained which has a markedly low S/N ratio and cannot be utilized practically. This is because the output power of the reproduced signal is proportional to the amplitude of the RF voltage. By way of example, in ordinary electrostatic capacity types of video disk systems, the amplitude of the RF voltage applied to the disk is set at approximately 4 V with the S/N ratio of the reproduced signal being taken into consideration.
As described above, even if the amplitude of the RF voltage applied to the information recording medium is set at the minimum value, i.e., approximately 1 V, such that the S/N ratio of the reproduced signal may not become markedly low, the setting value is much larger than 0.1 V, and therefore the information recorded on the information recording medium will be lost.
For the purposes of manufacturing an apparatus for reproducing the recorded information from the information recording medium of this type, it is desirable that the electroconductive head conventionally employed in the VHD system is utilized directly. However, with the conventional information recording medium in which depletion layers are formed, a degree of modulation .DELTA.C, i.e., the change in the electrostatic capacity caused to occur by the presence or absence of the depletion layers, is small, and therefore the conventional reproducing head cannot be utilized directly.
Also, the system for recording the information on the information recording medium by applying a voltage to the ferroelectric substance layer from the exterior in the manner described above, the problems occur in that, if the recording voltage is excessively high, the ferroelectric substance layer will suffer from dielectric breakdown, and the information recording medium will break. The intensity of the electric field, at which dielectric breakdown occurs, varies for different kinds of materials, structures, conditions, and the like, and is ordinarily approximately 1 MV/cm. The thickness of a recording layer of the information recording medium in accordance with the present invention is approximately 1 .mu.m or less. Therefore, in order for the information recording medium to be prevented from breaking due to dielectric breakdown, it is desirable that the voltage applied from the exterior during the recording step is kept at 100 V or less. However, such a recording method has not heretofore been developed.